1. Field of the Invention
This invention relates to thermoplastic condensation polymers which are polycarbonate-polysiloxane block copolymers and to methods of their preparation.
2. Brief Description of Related Art
The preparation of polycarbonate and polyestercarbonate resins from monomer reactants by the step growth or interfacial polycondensation procedure is well known; see for example the U.S. Pat. Nos. 3,169,121 and 4,188,314. Generally, a catalyst is employed to accelerate the rate of polymerization of the dihydric phenol reactant with the carbonate precursor such as phosgene and with the dicarboxylic acid (ester precursor) when employed. Catalysts commonly employed are the tertiary amines, including quaternary ammonium salts; see U.S. Pat. Nos. 3,240,755 (Col. 5, lines 65-74) and 3,240,756. (Col. 1, lines 26-27). In these descriptions, the quaternary ammonium salts are equated to, for example, trialkylamines as effective catalysts to promote the polymerization rate of the monomer reactants.
More recently, it was shown that phase transfer catalysts, can be used in place of, or in addition to tertiary amines to prepare high molecular weight polycarbonates employing reduced amounts of phosgene reactant (see U.S. Pat. No. 5,391,692). Polymerizations with a phase transfer catalyst present requires from 5 to 10% less phosgene to obtain a complete molecular weight build. Because complete molecular weight build was achieved with either a tertiary amine or a phase transfer catalyst, sight excesses of phosgene over and above the stoichometric amount did not change the weight average molecular weight.
In any event, the patentees in U.S. Pat. No. 5,391,692 did not consider the difficulty of preparing siloxane-polycarbonate block copolymers which contain high concentrations of the siloxane block with consistent and predictable weight average molecular weights. In addition, it was not known that the use of a phase transfer catalyst could be used in a process that would give both good phosgene utilization and block copolymers with consistent and predictable weight average molecular weights.
The preferred block copolymers prepared by the method of the invention are eugenol end-capped polydimethylsiloxane-bisphenol-A derived polycarbonate block copolymer, useful as a blend stock for blending with polycarbonate homopolymer resins to obtain polycarbonates with modified lower temperature modified impact resistance properties. Also, a high weight percent siloxane concentrate for blending with polycarbonates reduces the production costs of preparing such block copolymer in two ways: first, less block copolymer has to be made if it is a polysiloxane high level concentrate and second, because there is less of the polycarbonate block in the copolymer, residence time in the preparative reactions, is dramatically reduced.
We have found that we can prepare 80% eugenol-capped polydimethylsiloxane/20% polycarbonate ratio block copolymer concentrates consistently with the use of a phase transfer catalyst to catalyze their copolymerization. This catalyst gives consistent weight average molecular weight control, where previously used, catalysts such as triethylamine did not produce consistent or predictable molecular weights, regardless of the endcap levels used. Indeed, some polymerizations using triethylamine catalyst never stopped building molecular weight. The phase transfer catalyzed reaction also shows a dramatic reduction in phosgene usage (.ltoreq.25%) at the high Si incorporation levels. The eugenol capped polydimethylsiloxane block copolymers show improved advantageous utility as a blend ingredient for use in preparing polycarbonate blends.